Th17 cells denote a recently identified T lymphocyte helper subset that is proinflammatory and is involved in the pathogenesis of autoimmune dyscrasias. B1 cells represent a distinct B lymphocyte lineage and fundamental component of the immune system that is responsible for the production of natural immunoglobulin. Recent published work from this laboratory has shown that B1 cells are uniquely efficient in stimulating naove T cells to differentiate into Th17 cells, whereas B2 cells do so only poorly or not at all. The long term objective of this proposal and beyond is to elucidate the role and function of B1 cells within the immune system in terms of B cell/T cell interaction. The goals of the work described herein relate to the remarkable capacity of B1 cells to induce Th17 cell differentiation. The specific aims are to 1) Determine how B1 cells function effectively to induce Th17 cell differentiation when B2 cells do not, by evaluating the potential role of cytokines and retinoic acid, through assay of supernatants by ELISA, supplementation of B/T cultures with recombinant proteins, and reduction of cytokine levels via neutralizing antibodies and knock-out animals;2) Determine how B1 cells function effectively to induce Th17 cell differentiation when B2 cells do not, by evaluating the potential role of B7 family members, specifically CD80, CD86, and PD-L2, through reduction of B7 co-stimulatory interactions via neutralizing antibodies and knock-out animals, and by partial replacement of B7 family members in deficient mice through construction of retrovirally transduced bone marrow chimeras;3) Test the in vivo activity of B1 cells in producing Th17 cell differentiation using OVA-specific OT-II T cells adoptively transferred to Class II-deficient mice along with B cells and OVA peptide and protein. The results of this study are expected to provide new information about the function of B1 cells and new information about the factors that induce Th17 cell differentiation, as well as demonstrate that B1 cells can direct T cell differentiation in vivo. The results generated as a result of this project will illuminate the new and unexpected connection between B1 cells and Th17 cells, furthering knowledge of the role of B1 cells during interaction with T cells and reflecting on the place and activity of B1 cells within the immune system. In terms of the latter, the work described here may delineate a new role for B cells wherein distinct B cell lineages drive specific T cell polarization, placing B cells as the directors/managers of T cell function. Elucidation of these points is likely to provide new targets and strategies to ameliorate the progression of autoimmune dyscrasias. Public Health Relevance: Th17 cells are a recently discovered subpopulation of T lymphocytes that produces inflammation and that is intimately involved in autoimmune diseases. B1 cells are a subpopulation of B lymphocytes that are especially valuable because they produce disease-fighting antibody without the need for vaccination, but they are problematical because they too, like some T cells, are involved in autoimmune diseases. The work described in this proposal builds on the new discovery that B1 cells are especially good at activating Th17 cells to produce inflammation. The work outlined in this project is designed to learn more about this new function of B1 cells, by figuring out why B1 cells are so effective in stimulating resting T cells to become inflammatory Th17 cells, by determining whether B1 cell-induced inflammatory Th17 cells are involved in producing autoimmunity, and by gauging the ability of B1 cells to produce inflammatory Th17 cells in animal models. These studies will provide information on how to regulate the number of inflammatory cells in the body;this information might then be useful in autoimmune treating diseases like rheumatoid arthritis.